We have all heard the term Specific Gravity and we use it all the time, but what is it? Why is it so important for the brewing process? The dictionary tells us that SG is the ratio of the density of a substance to the density of a reference substance; in this case it would be ratio of wort density to water density.

But what does that mean? What are we actually measuring? How does it relate to other values such as density, Brix, Plato, Solids, Refraction, etc? Specific Gravity is actually an indirect way of measuring wort sugar concentration (extract) and/or beer alcohol content using a very simple measuring device.

As starches are extracted from grain and converted into sugars, the physical and chemical properties of the water/liquor change as it becomes wort. We measure the changes in the properties of water/wort to learn about concentration of extract and/or alcohol content.Among these properties there are three that are somewhat related and can be measured to achieve the same goal:

Density: it represents the weight of a specific volume of something; water at 40°F has a density of 8.344 pounds per US-gallon. Since we now know that SG is the known ratio between the density of wort and the density of water, we can easily infer that the density of, for example, wort with an OG of 1.050 is 8.76 pounds per US-gallon (8.344 x 1.05 = 8.761).

Dry matter (AKA solids): represents the wort constituents excluding water such as starches, sugars, proteins, fats, etc. By measuring either the dry matter or water content of wort we are actually measuring the amount of extract. Solids can be measured with the oven method, but it requires an expensive analytical scale.

Refraction: When light moves from one medium to another one (from air to wort), it changes direction. This is, in my opinion, one of the easiest and cleanest way to measure wort concentration. Using a hand-held, laboratory or in-line refractometer, the refraction angle of wort can be easily measured and correlated to Brix.

Now that you are loaded up with information and you are getting ready to start your next batch of beer, you may be asking, how is this going to help me?

Either when brewing at home or in small, medium or large commercial breweries accurate measurement and control of the variables that go into brewing is crucial for the production of high-quality beers CONSISTENTLY. And here’s where all this information becomes useful.

Commercial breweries have typically at their disposal an army of sensors and “gadgets” to measure wort concentration both during the different steps brewing process (mashing, boiling and fermenting) and at the laboratory. Inline or in-tank refractometers, oscillating utubes, pressure transducers and turbidity meters represent a few of the devices available for commercial breweries. But how can a home brewer achieve the level of precision that commercial brewing operations have?

Home brewers have access to several simple instruments and techniques that can be used to achieve the same level of precision. Of course the Hydrometer (AKA density meter, alcoholmeter, saccharometer…) is the most common device to measure SG at home or any size brewery, but as a paranoid home brewer, I see it as a potential source of contamination of my precious beer, especially during fermentation.

After the boil, during fermentation, the beer is susceptible to contamination by wild yeasts or bacteria (infection) and every effort should be made to prevent such contamination.But how then can we measure the progress of fermentation without risking the integrity of the beer? Commercial breweries have access to their wort/beer through sampling valves without affecting the integrity of the beer, so what can we home brewers use?

The first device that comes to my mind is a hand-held refractometer ($150-$200). This device uses a single drop of liquid to measure the angle of refraction and uses an internal chart to display the Brix value which in turn can be correlated to SG. Using a sanitized spoon, or plastic stick, a single drop of the beer can safely be obtained from the fermenter and placed on the refractometer to obtain an immediate answer. (When using a refractometer brewers and mead makers can then use the Brix value on the Pearson Square to adjust the SG before fermentation.)

I think that with some creativity home brewers can come up a wide range of “my-ownmeters” to track the progress of the fermentation without compromising the integrity of the beer. Alewives used to put chestnuts in the beer, and depending on whether they sank or floated, they would have an indication of the progress of the fermentation. But in this case the properties of the chestnuts change overtime, and adds potential for infection.

There might be a case for ice-cubes prepared with sugar solutions of different concentrations.You would simply drop the ice-cube with the target SG into the fermenter and easily evaluate the progress of the fermentation… Or you could use a graduated fermenter mounted on a scale to track the weight and volume and calculate the density and SG.

After all this, now comes the issue of the units, let alone the issue between US and Imperial gallons and liters, how about things like Baume Scale, Brix Scale, Oechsle Scale (wine), °Plato, ABV, etc.? Thankfully in this case things are a lot simpler since most of them refer to the same thing in slightly different ways.

The Plato Scale is used to measure the density of beer wort as a percent of extract by weight: 1° Plato = 1% Brix = 1% sucrose by weight.

The conversion from ºPlato to SG is not quite linear (the formula looks scarier than it actually is) but in the SG range of most beers(1. 00 to 1.15) the formula can be approximated to:

I Recently read that abstract math symbols stimulate the organ that secretes eye-glaze juice; hopefully this won’t be that bad.

For example, to convert a Specific Gravity of 1.050 to °P:

• (1.050 – 1) / 0.00411 = °P

• 0.050/0.00411 = °P

• 12.2°P For those of you interested, you could easIly enter the following formula on a spreadsheet.Column A for SG and on column B for °P: =(A1-1)/0.00411

But wait… what if I have the ºPlato value and I want to find out the specific gravity?In this case the spreadsheet formula is as follows. Column A for °P and on Column B for SG: =A2/(258,6-(0,8795*A2))+1

Now that your beer SG has stabilized after a few weeks in the fermenter, now is time to calculate the alcohol by volume (ABV) of the finished product. For this we use the difference between the Original Specific Gravity OG (pre-fermentation) and the Final Specific Gravity FG (post fermentation), as well as the density of ethanol at 65°F and the amount of ethanol produced per amount of CO2. Most brewers simply multiply the difference between OG and FG times 1.33.

If you are interested in a more accurate calculation, you can enter the following formula on a spreadsheet, A column for OG, B column for FG and on C column: =(1.05*(A1- B1))/B1/0.79 (make sure you format the cell to display %.)

I hope you find the information easy to digest and useful, happy brewing.